Chameleons (family Chamaeleonidae) are a distinctive and highly specialized clade of lizards. They are distinguished by their parrot-like zygodactylous feet, their separately mobile and stereoscopic eyes, their very long, highly modified, and rapidly extrudable tongues, their swaying gait, and the possession by many of a prehensile tail, crests or horns on their distinctively shaped heads, and the ability of some to change color. Uniquely adapted for climbing and visual hunting, the approximately 160 species of chameleon range from Africa, Madagascar, Spain and Portugal, across south Asia, to Sri Lanka, have been introduced to Hawaii and California, and are found in warm habitats that vary from rain forest to desert conditions.

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The English word chameleon (also chamaeleon) derives from the Latin chamaeleo which is borrowed from the Ancient Greek χαμαιλέων (khamaileon), a compound of χαμαί (khamai) "on the earth, on the ground" + λέων (leon) "lion". The Greek word is a calque translating the Akkadian nēš qaqqari, "ground lion".[1]

Chameleons vary greatly in size and body structure, with maximum total length varying from 3.3 cm (1.3 in.) in Brookesia minima (one of the world's smallest reptiles, possibly only surpassed by geckos from the genus Sphaerodactylus) to 68.5 cm (27 in.) in the male Furcifer oustaleti.[2] Many have head or facial ornamentation, such as nasal protrusions, or horn-like projections in the case of Chamaeleo jacksonii, or large crests on top of their head, like Chamaeleo calyptratus. Many species are sexually dimorphic, and males are typically much more ornamented than the female chameleons.

Chameleon species have in common their foot structure, eyes, lack of ears, and tongues.

Chameleons are didactyl: on each foot the five toes are fused into a group of two and a group of three, giving the foot a tongs-like appearance. These specialized feet allow chameleons to grip tightly to narrow branches. Each toe is equipped with a sharp claw to gain traction on surfaces such as bark when climbing. The claws make it easy to see how many toes are fused into each part of the foot — two toes on the outside of each front foot and three on the inside.

Their eyes are the most distinctive among the reptiles. The upper and lower eyelids are joined, with only a pinhole large enough for the pupil to see through. They can rotate and focus separately to observe two different objects simultaneously. It in effect gives them a full 360-degree arc of vision around their body. When prey is located, both eyes can be focused in the same direction, giving sharp stereoscopic vision and depth perception. They have very good eyesight for reptiles, letting them see small insects from a long (5-10 cm) distance.

They lack a vomeronasal organ. Also, like snakes, they do not have an outer or a middle ear. This suggests that chameleons might be deaf, although it should be noted that snakes can sense vibration using a bone called the quadrateim. Furthermore, some or maybe all chameleons, can communicate via vibrations that travel through solid material like branches.

Chameleons have very long tongues (sometimes longer than their own body length) which they are capable of rapidly extending out of the mouth. The tongue extends out faster than human eyes can follow, at around 26 body lengths per second. The tongue hits the prey in about 30 thousandths of a second.[3] The tongue of the chameleon is a complex arrangement of bone, muscle and sinew. At the base of the tongue there is a bone and this is shot forward giving the tongue the initial momentum it needs to reach the prey quickly. At the tip of the elastic tongue there is a muscular, club-like structure covered in thick mucus that forms a suction cup.[4] Once the tip sticks to a prey item, it is drawn quickly back into the mouth, where the chameleon's strong jaws crush it and it is consumed. Even a small chameleon is capable of eating a large locust or mantis. Ultraviolet light is part of the visible spectrum for chameleons.[5] Chameleons exposed to ultraviolet light show increased social behavior and activity levels, are more inclined to bask and feed and are also more likely to reproduce as it has a positive effect on the pineal gland.

Chameleons are primarily found in Africa and Madagascar, and other tropical regions, although some types are also found in parts of southern Europe, the Middle East, southern India, Sri Lanka and several islands in the western Indian Ocean. There are introduced, feral populations of veiled and Jackson's chameleons in Hawaii and isolated pockets of feral Jackson's chameleons have been reported in California and [Florida.

Chameleons inhabit all kinds of tropical and mountain rain forests, savannas and sometimes semi-deserts and steppes. They are mostly arboreal and are often found in trees or occasionally on smaller bushes. Some smaller species live on the ground under foliage.

The oviparous species lay eggs after a 3-6 week gestation period. The female will climb down to the ground and begin digging a hole, anywhere from 10-30 cm (4-12 in.) deep depending on the species. The female turns herself around at the bottom of the hole and deposits her eggs. Once finished, the female buries lina

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and leaves the nesting site. Clutch sizes vary greatly with species. Small Brookesia species may only lay 2-4 eggs, while large Veiled Chameleons (Chamaeleo calyptratus) have been known to lay clutches of 80-100 eggs. Clutch sizes can also vary greatly among the same species. Eggs generally hatch after 4–12 months, again depending on species. The eggs of Parson's Chameleon (Calumma parsonii), a species which is rare in captivity, are believed to take upwards of 24 months to hatch.

The ovoviviparous species, such as the Jackson's Chameleon (Chamaeleonatisozianapus jacksonii) have a 5-6 month gestation period. The newborn are in a transparent membrane and they are still sleeping, once they touch the ground or branch, they will wake up and attempt to crawl out of the membrane.[citation needed] The female can have 8-31 live young at once. Their is 291 billion left in the world but its an endangered species.

Color change is also used as an expression of the physiological condition of the lizard, and as a social indicator to other chameleons and it is thought that these change because they get a message from the brain. The message tells the cells to enlarge or to shrink. These actions cause cell pigments to mix-just like paint. Some research suggests that social signaling was the primary driving force behind the evolution of color change, and that camouflage evolved as a secondary concern.[9][10]

Chameleons have specialized cells, collectively called chromatophores, that lie in layers under their transparent outer skin. The cells in the upper layer, called xanthophores and erythrophores, contain yellow and red pigments respectively. Below these is another layer of cells called iridophores or guanophores, and they contain the colorless crystalline substance guanine. These reflect, among others, the blue part of incident light. If the upper layer of itchromatophores appears mainly yellow, the reflected light becomes green (blue plus yellow). A layer of dark melanin contained in melanophores is situated even deeper under the reflective iridophores. The melanophores influence the 'lightness' of the reflected light. These specialized cells are full of pigment granules, which are located in their cytoplasm. Dispersion of the pigment granules in the cell grants the intensity of appropriate color. If the pigment is equally distributed in the cell, the whole cell has the intensive color, which depends on the type of chromatophore cell. If the pigment is located only in the center of the cell, cell appears to be transparent. All these pigment cells can rapidly relocate their pigments, thereby influencing the color of the chameleon.